1. Field of the Invention
The present invention relates to pipe freezing apparatus, and in particular to a valve or switching unit for controlling the flow of refrigerant through a freezer head.
2. Description of the Related Art
EP-A-0145114 describes a pipe freezing apparatus which is particularly suited to freezing water in pipes, to enable a pipe to be cut without draining the system. A freezer head is attached to the pipe to each side of the region to be worked on, and plugs of frozen water are formed in the pipe, at the location of the freezer heads. The freezer heads constitute expansion chambers, liquid refrigerant being pumped (or drawn) into the chamber and evaporating in the chamber to cool the chamber walls. This arrangement is well known.
Typically a refrigeration system is controlled by a thermostat or pressure switch which turns the compressor off at the required setting. An adjustable thermostatic expansion device may also be used. These controls are not suitable or not convenient for pipe freezing apparatus because the freezer heads are on flexible hoses and at some distance from the compressor.
Some known types of pipe freezing apparatus require periodic manual operation thereof in order to effectively freeze the liquid in a pipe. Thus, for example, it may be necessary to release refrigerant into the freezer head (or jacket surrounding the evaporation chamber) for 90 seconds and then a further 60 seconds after a delay of 2 minutes. Further variable periods of refrigerant release may be required after varying delay times have elapsed, depending upon many factors, including pipe diameter. In fact, this sequence of varying periods required to effect a freeze, can take a relatively long time, during which an operator must be in constant attendance.
Furthermore, after the freeze has been effected, and an ice plug has been formed, some types of pipe freezing apparatus require the operator to return to the freezer head (or jacket) to release an extra burst of refrigerant, otherwise the ice plug may thaw. This manual maintenance of the ice plug is again time consuming and often necessitates inconvenient interruptions of an operator's work schedule.